1. Field of the Invention
The present invention relates to a gas flow path switching unit suitable for a multidimensional gas chromatograph.
2. Description of the Related Art
In a multidimensional gas chromatograph that uses a combination of a plurality of columns with different separation characteristics and a plurality of detectors arranged side by side, a gas flow path switching unit is essential in order to allow a sample gas introduced via a certain passage to be selectively sent to a desired one of two or more branch passages. If the gas flow path switching unit is designed such that a movable portion of a three-way valve or the like resides inside a gas passage, it will face problems, such as increase in dead volume, and absorption of components of a sample gas by a material (e.g., grease) for providing enhanced movability to the movable portion, or, on the contrary, incorporation of such an undesirable material into the sample gas. From this point of view, a specific structure, called “Deans type”, has been used as the gas flow path switching unit (see, for example, JP 11-248694A, JP 2000-179714A and JP 2007-187663A).
FIGS. 8A and 8B are schematic diagrams showing a basic structure of a Deans-type gas flow path switching unit. In this unit, a main passage 71 having one end serving as an gas inlet 70 for allowing a sample gas to be supplied therethrough is branched into a first branch passage 74 directed toward a gas outlet A, and a second branch passage 73 directed toward a gas outlet B, at a branch point 72. A first switching-gas supply passage 75 is connected between an intermediate position of the first branch passage 74 and one of two exits of a three-way switching valve 77, and a second switching-gas supply passage 76 is connected between an intermediate position of the second branch passage 73 and the other exit of the three-way switching valve 77. Further, a resistance tube having a given flow resistance is connected between the first and second switching-gas supply passages 75, 76. A switching-gas source passage 79 having a pressure control valve 80 interposed therein is connected to an entrance of the three-way switching valve 77.
The three-way switching valve 77 is composed of a solenoid valve, and is operable to selectively connect either one of the first and second switching-gas supply passages 75, 76 to the switching-gas source passage 79. The pressure control valve 80 is provided with a pressure sensor for detecting a gas pressure on a downstream side of the pressure control valve 80, and designed such that a valve opening degree thereof is automatically adjusted to allow the gas pressure to be maintained at a predetermined value.
An operation of the Deans-type gas flow path switching unit will be described below. Under control of the pressure control valve 80, the gas pressure on the downstream side thereof is now maintained at P1. In a state when the switching-gas source passage 79 is connected to the second switching-gas supply passage 76 by the three-way switching value 77, a switching gas flows as indicated by the arrowed dotted-lines in FIG. 8A. Given that a flow resistance in each passage is zero, and a pressure drop during passing through the resistance tube 78 is ΔP, a gas pressure on the side of the gas outlet B becomes P1, and a gas pressure on the side of the gas outlet A becomes P1−ΔP. A sample gas supplied from the gas inlet 70 to the main passage 71 flows from the branch point 72 into one of the branch passages 73, 74 which has a lower pressure. Thus, the sample gas is discharged from the outlet A via the first branch passage 74, as indicated by the arrowed thick-lines in FIG. 8A.
Then, when the three-way switching value 77 is switched to connect the switching-gas source passage 79 to the first switching-gas supply passage 75, the switching gas flows as indicated by the arrowed dotted-lines in FIG. 8B, so that the gas pressure on the side the outlet A becomes P1, and the gas pressure on the side of the outlet B becomes P1−ΔP. Therefore, as indicated by the arrowed thick-lines in FIG. 8B, the sample gas supplied from the gas inlet 70 to the main passage 71 is discharged from the outlet B via the second branch passage 73.
As above, the Deans-type gas flow path switching unit is designed to change a flow direction of the switching gas by the three-way switching value 77, so as to allow an outlet of the sample gas introduced from the gas inlet 70 to be switched between the gas outlets A, B.
Lately, as gas chromatographic analysis techniques have progressed, there has been a demand for a system capable of introducing an elution component from an analytical column, into any of three or more different detectors, such as a mass spectrometer, a hydrogen flame ionization detector (FID), a flame photometric detector (FPD), and an electron capture detector (ECD), in an appropriately switchable manner. In order to meet this demand, a gas flow path switching unit must be capable of allowing a sample gas to switch between any of three or more flow paths. However, if it is attempted to achieve the switching between the three or more flow paths using the conventional Deans-type gas flow path switching unit, the number of components and unit cost are undesirably increased, and other problems, such as failure, gas leakage and absorption of components of a sample gas, are liable to occur.